As you are in Kalasin pop to immigration there & ask is the best option. But saying that I did for my last extension & submitted paperwork & they tossed
You don’t really need a Thai embassy city just leave return on tourist then extend & convert to O in country. But I would get exact details from immigration as time line etc. I did this a few years back changing from a O/A to a O flew to Nong Khai jumped on a bus & zipped over & back no problem